A network manager collects and analyzes traffic of a network, for example, periodically in order to detect an abnormal state of the network. The network manager detects, for example, a sudden increase of the traffic as an abnormal state of the network. In order to suppress congestion of communication or failure of a specific apparatus by a sudden increase of the traffic, the network manager performs installation of a new switch or control of a communication path for distributing the traffic. As related documents, there are International Publication Pamphlet No. WO 2012/127894, Japanese Laid-open Patent Publication No. 2002-16599, and Japanese Laid-open Patent Publication No. 2004-343203.